IT. Expert System.

 Network Working Group                                          J. Postel
Request for Comments: 862                                            ISI
                                                                May 1983
                             Echo Protocol
This RFC specifies a standard for the ARPA Internet community.  Hosts on
the ARPA Internet that choose to implement an Echo Protocol are expected
to adopt and implement this standard.
A very useful debugging and measurement tool is an echo service.  An
echo service simply sends back to the originating source any data it
receives.
TCP Based Echo Service
   One echo service is defined as a connection based application on TCP.
   A server listens for TCP connections on TCP port 7.  Once a
   connection is established any data received is sent back.  This
   continues until the calling user terminates the connection.
UDP Based Echo Service
   Another echo service is defined as a datagram based application on
   UDP.  A server listens for UDP datagrams on UDP port 7.  When a
   datagram is received, the data from it is sent back in an answering
   datagram.
Postel                                                          [Page 1]